1. Field of the Invention
This invention relates to medical imaging systems using nuclear magnetic resonance. In a primary application the invention relates to providing NMR imaging systems which are relatively immune to variations in the magnetic fields.
2. Description of Prior Art
Nuclear magnetic resonance, abbreviated NMR, represents a new approach to medical imaging. For general descriptions of the various imaging methods we can use a number of references including the books, NMR Imaging in Biomedicine by P. Mansfield and P. G. Morris, Academic Press and Nuclear Magnetic Resonance Imaging in Medicine, published in 1981 by Igaku-Shoin, Ltd. Tokyo or review papers including "NMR Imaging Techniques and Applications: A Review," by P. A. Bottomley, Rev. Sci. Instrum., vol. 53, September 1982, pp. 1319-1337, and "Fourier Transform Nuclear Magnetic Resonance Tomographic Imaging," by Z. H. Cho, et al., Proceedings of the IEEE, vol. 70, October 1982, pp. 1152-1173. In these, various NMR imaging systems are described, each requiring a highly stable magnetic field because of the critical nature of the magnetic resonance. Subtle variations in these fields can cause either severe distortions or complete loss of the image. As a result of these severe stability requirements, most manufacturers have begun to use superconducting magnets, which are both expensive and difficult to maintain. They are also incapable of reasonably rapid field variations, which is desirable for some studies such as relaxation times as a function of frequency.
This problem of a severe stability requirement was addressed in UK Application No. GB 2076542A and U.S. Pat. No. 4,417,209 by Godfrey N. Hounsfield. In this patent small pickup coils are used to sample the received signals on either side of the object being studied. The output from these coils form the demodulating signals for the signals received from the principal receiver coil. Thus magnetic field variations during the time signals are received are compensated for.
This approach, however, does not compensate for field variations which cause errors in the transmitter or excitation signals. Also, the method shown requires complex handling of the signals when projections are taken in different directions. At least four separate pickup coils are required, preferably a pair for each projection angle, or a single pair of pickup coils which are rotated for each projection angle.
It would be highly desirable to have a self-compensating NMR imaging system which tolerated magnetic field changes. In U.S. application Ser. No. 457,589 now U.S. Pat. No. 4,521,734 by the same inventor entitled, "Pulsed Main Field Nuclear Magnetic Resonance Imaging System," an improved system of supplying the main magnetic field is described, using a pulsed electromagnet. This provides higher fields at reduced dissipation, lower cost, and flexibility of the magnetic field. It does have the disadvantage, however, of being potentially susceptible to variations in the magnetic field. A system which is self-compensating would solve this problem, and provide for a greatly improved and more flexible system.
In U.S. application Ser. No. 476,474 filed 3/18/83 a basic method was shown of compensating for magnetic field changes. A two-burst excitation scheme was used, such as inversion recovery or spin echo. The first excitation was a nonselective broadband burst, with the second excitation a selective narrow-band burst, all in the presence of a gradient field. The signal received from a reference coil around a reference volume, during the first burst, is used to estimate the frequency required for the selective burst. Thus the desired region is excited independent of drift. Similarly, during the receive mode, reference coils on either side of the object measure the frequency and bandwidth of the received signals. These signals are used to produce demodulation and sampling signals. In this approach the reference volumes are excited by the same excitation field as that of the object.